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WEDNESDAY, April 3, 2013 (MedPage) — Chinese authorities are stepping up surveillance after reporting seven cases of H7N9 influenza A, the first time people have been known to be infected with the avian flu strain.

Three cases were reported to the World Health Organization on March 31 and another four cases were reported in China Tuesday, according to the official Xinhua news agency.

Two of the first three patients have died, and the other is in critical condition in hospital. There have been no deaths among the next four, Xinhua reported, but all are in intensive care.

Thus far there is no evidence of human-to-human transmission, Chinese and WHO officials say, and it remains unclear if the current outbreak is isolated or the beginning of a wider epidemic.

Flu viruses are named according to the variants of surface proteins they carry — one of 16 types of hemagglutinin (H) and one of 9 forms of neuraminidase (N).

Among the various types of avian influenza, the highly pathogenic H5N1 strain get the most press, largely because of its staggering mortality rate when it infects humans — 371 deaths out of the 622 cases since 2003. On the other hand, such infections are rare and usually occur in people in close contact with infected poultry.

But, until now, H7N9 viruses have not been known to infect people, according to the WHO. Other H7 viruses, however, have caused outbreaks, although they have usually caused no more than mild upper respiratory disease and conjunctivitis.

According to the Chinese Center for Disease Control and Prevention, H7 viruses have not previously caused human disease in China and H7N9 in particular had only been previously isolated in birds.

So far, there are no apparent epidemiological links among the seven cases and the Chinese CDC reported that no infections were found among 88 close contacts of the first three cases.

Two of those victims came from Shanghai and one in Anhui, a province west of Shanghai. The remaining four were reported in the province of Jiangsu, north of Shanghai.

According to Xinhua, two of those patients — a 45-year-old woman who worked as a poultry culler and a 48-year-old female sheet metal worker — developed fever, aches, and respiratory symptoms on March 19 and were admitted to hospital on March 27 and 20, respectively.

An 83-year-old man developed fever and respiratory symptoms on March 20 and was admitted to hospital on March 29, while the fourth patient -- a 32-year-old unemployed woman – came down with cough, fever, and other symptoms on March 21 and was admitted to hospital March 28.

The Hong Kong Centre for Health Protection said in a statement that so far no epidemiological links have been found among the four patients and no other H7N9 infections have been found in 167 of their close contacts.

Interestingly, a recent study highlighted the Shanghai region — the coastal provinces bordering the South China Sea and East China Sea — as a possible flu "hot spot" where different strains of influenza could mix and mingle genetically with the potential to create new human-adapted disease.

Chinese researchers have already sequenced isolates from the first three cases and published them on GISAID, a publicly available database of flu genetic sequences.

Using those data, researchers are racing to see what has changed in the H7N9 lineage that might make it jump from birds to humans. Indeed, some researchers are already posting their early analyses online.

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